1. Field of the Invention
The field of the invention is that of mailing and in particular that of mailing systems capable of processing a large number of mail items at high speed.
To be more precise, the invention concerns the franking of mail in such systems.
2. Description of the Prior Art
As a general rule, known types of high-speed mailing systems comprises at least one inserter machine and a franking machine. The inserter machine folds the enclosures to be mailed and inserts them in the envelopes. It feeds the envelopes one by one to the franking machine, also called a printing machine, which prints various information such as mailing symbols (postage stamp and office stamp) and promotional material.
The invention is therefore concerned with these franking machines and more precisely with optimizing their operation and in particular the franking speed. In other words, the invention concerns an optimized control device for a printing machine which is part of a mailing system.
A printing machine usually comprises the following parts:
a print drum incorporating the print head (carrying the information to be printed on the envelopes); PA1 a security system often comprising an assembly including an obturator bar locking the print drum drive system; PA1 conveyor means which deliver envelopes from the inserter device to the franking station of the printing machine and eject the envelopes after the print head has printed the mailing symbols and the promotional material. PA1 conveying mail items, PA1 rotating the drum of the franking machine, PA1 moving the obturator bar of the franking machine, are controlled by a single microcomputer providing centralized control of the various actuators. PA1 the printing speed of the print drum is the minimum speed required, PA1 during each print cycle, the conveyor speed is equal: PA1 during each cycle, the speed of the print drum and therefore that of the print head is: PA1 printing means comprising a rotary print head actuated by a first motor and carrying on a portion of its surface a printing active part; PA1 conveyor means for conveying said envelopes actuated by a second motor and feeding said envelopes into contact with said printing means at a given conveyor speed and for evacuating franked envelopes; PA1 means for optimizing the rotation speed of said first motor operative such that the tangential speed of said print head is held equal to said conveyor speed during a printing phase corresponding to the period during which an envelope is in contact with said active printing part and as close as possible to said conveyor speed during a complementary catch-up phase; and PA1 means for optimizing said conveyor speed operative such that said conveyor speed is as low as possible whilst preventing overlapping of envelopes arriving on said conveyor means. PA1 determining the rate of arrival of documents on said conveyor means, PA1 calculating a catch-up speed to be applied to said first motor during said catch-up phase according to said document arrival rate, PA1 determining the difference between said catch-up speed and said conveyor speed, PA1 correcting said conveyor speed if said speed difference is greater than a predetermined threshold.
The mailing symbols and the promotional material are printed as the print drum moves at a constant speed sometimes called the "printing speed".
It is obvious that to print correctly during this printing phase the tangential speed at the surface of the drum must be equal to that of the mail item at the printing location. This is achieved if the "conveyor speed" at which the conveyor means deliver the envelopes from the inserter machine is equal to the printing speed throughout the printing phase.
The rest of the time, that is to say when there is no printing taking place, the drum is usually stopped. When an envelope is fed in the drum starts and accelerates up to the printing speed.
This first type of printing machine has the major drawback of restricting the throughput of the mailing system as a whole because during each printing cycle (to frank an envelope) the print drum must be stopped completely and the security system engaged and disengaged.
The print drum must run up from zero speed to the printing speed before printing the symbols and is then decelerated to a stop after printing. The obturator bar must then be returned to the rest position (the position which disables printing).
Consequently, to increase the overall system throughput the inserter machine must increase the speed with which mail is ejected, which involves increasing the printing speed of the print drum. The magnitudes of the print drum deceleration and acceleration are increased accordingly.
A conventional solution to this problem is to use more powerful motors to obtain a higher speed and a higher torque. Apart from increasing the physical size of the machine, this increases the power consumption and the heat dissipation, which increases the cost of the system.
Also, operation of the franking machine at a higher speed and with higher acceleration and deceleration leads to premature wear of the machine and its drive system.
The higher the speed, the harder the acceleration and deceleration and the faster the movement of the obturator, which causes numerous impacts harmful to the mechanical parts.
These impacts cause a high noise level in operation which is very uncomfortable for the mailing system user and others in the immediate environment, especially in the event of regular or especially continuous use.
A second type of printing machine has been designed with the aim of increasing the throughput of a mailing machine incorporating a franking machine whilst reducing the drawbacks in terms of wear of the system as explained above. This machine is described in the patent document U.S. Pat. No. 2,619,643.
In this second type of printing machine three independent drive mechanisms respectively:
Means for sensing the entry speed of the envelopes and means for sensing triggering according to the position of the envelope supply data to the microcomputer which controls the various mechanisms of the machine so that:
to the printing speed during printing, and PA2 to the envelope arrival speed (greater than the printing speed) before deceleration and after acceleration, PA2 equal to the printing speed during printing, and PA2 zero before acceleration and after deceleration to enable positioning of the obturator.
By simultaneously controlling the envelope arrival rate and the speed of the drum these machines are able to provide a higher throughput than machines of the first type described above. However, the need to stop the drum completely during each cycle remains a factor limiting the throughput of the mailing system as a whole.
With this type of machine it is necessary to manipulate an obturator during each cycle. The machine is therefore still subject to sudden stopping of the print head drum and its drive system which inevitably causes vibration, wear and noise.
Finally, to minimize the printing speed of the print drum in such machines the conveyor speed is caused to vary between a high value (the envelope entry speed, that is to say the speed of the envelopes on leaving the inserter machine) and a low value (the printing speed) which is required to be as low as possible. In other words the conveyor speed varies continuously between a printing phase and an envelope feeding phase. Controlling the various speeds is therefore complex and it is difficult to optimize these speeds.
The U.S. Pat. No. 4,023,489 describes a printing device in which the conveyor speed is constant and in which the tangential speed of a print drum varies between zero and the conveyor speed but does not return to zero if the documents to be printed arrive at a rate which is at or above a fixed value.
Consequently the magnitude of variations in the drum speed is usually reduced as compared with machines in which the drum is stopped in each cycle. However, this machine has the drawback that the amplitude of variation remains high when the arrival throughput is low in comparison with the maximal throughput allowed by the machine because the conveyor speed and therefore the speed of the drum during printing is fixed and remains equal to a value corresponding to the maximal throughput of the machine.
Furthermore, if a low conveyor speed is chosen it limits the throughput of the printing device unnecessarily.
A particular objective of the invention is to alleviate these various drawbacks of the prior art. To be more precise, an objective of the invention is to provide a printing machine control device enabling reduction of vibration and impact caused by operation of the drum, in particular to reduce wear and the operating noise of the machine, whilst allowing the throughput of the mailing system of which the printing machine is part to be increased.
Another objective of the invention is to provide a device of this kind which is simple, reliable and of low cost, especially in comparison with existing devices such as those described previously.
Another objective of the invention is to provide a device of this kind which enables optimized operation of the franking machine irrespective of the envelopes processed, their number and their rate of arrival.